The ability to manipulate individual atoms for use in nanotechnology components continues to develop. Some of these developments are in the field of materials and specifically atomically thin materials which may use a single molecular component or selected combinations of molecular components. One example of such a material is graphene which is a two-dimensional aromatic polymer that has a multitude of applications ranging from electronic memory, electrical storage, composite enhancement, membranes and the like. Other atomically thin materials are believed to have their own beneficial properties.
A graphene membrane is a single-atomic-layer-thick layer of carbon atoms, bound together to define a sheet. The thickness of a single graphene membrane, which may be referred to as a layer or a sheet, is approximately 0.2 to 0.3 nanometers (nm) thick, or as sometimes referred to herein “thin.” The carbon atoms of the graphene layer define a repeating pattern of hexagonal ring structures (benzene rings) constructed of six carbon atoms, which form a honeycomb lattice of carbon atoms. An interstitial aperture is formed by each six carbon atom ring structure in the sheet and this interstitial aperture is less than one nanometer across. Indeed, skilled artisans will appreciate that the interstitial aperture is believed to be about 0.23 nanometers across at its longest dimension. Accordingly, the dimension and configuration of the interstitial aperture and the electron nature of the graphene precludes transport of any molecule across the graphene's thickness unless there are perforations.
Recent developments have focused upon graphene membranes for use as filtration membranes in applications such as salt water desalination. One example of such an application is disclosed in U.S. Pat. No. 8,361,321 which is incorporated herein by reference. As these various uses of graphene and other atomically thin materials develop, there is a need to manufacture relatively large area graphene sheets for use in filtration applications and other uses.
Without considering the possibility of lattice defects, the carbon atoms in graphene, or other atoms in an atomically thin layer, are so closely spaced that a sheet or layer of the material is essentially impermeable to most substances. However, if holes with the proper dimensions are made in the layer, molecules smaller than these holes can readily pass through the layer. Molecules with dimensions larger than the holes will not be able pass through the layer. In the case of graphene, such a layer with properly sized holes is a “molecular filter,” and it can be used to separate molecules based on their size differences. With properly sized holes, a perforated graphene layer becomes a nano-filter or ultra-filter. Because of its extreme thinness, the energy cost for moving a molecule across such a molecular membrane is lower than other competing filtration membranes that rely on Solution-Diffusion mechanisms for separation.
Various methodologies are known to form nano-sized to micro-sized holes in an atomically thin layer such as graphene. It is known to form graphene apertures or holes by selective oxidation, by which is meant exposure to an oxidizing agent for a selected period of time. It is believed that graphene apertures can also be formed by charged particle bombardment thereafter followed by the aforementioned selective oxidation. As described in the publication Nano Lett. 2008, Vol. 8, no. 7, pg 1965-1970, the most straightforward perforation strategy is to treat the graphene film with dilute oxygen in argon at elevated temperature. As described therein, through apertures or holes in the 20 to 180 nm range were etched in graphene using 350 mTorr of oxygen in 1 atmosphere (atm) argon at 500° C. for 2 hours. The paper reasonably suggests that the number of holes is related to defects in the graphene sheet and the size of the holes is related to the residence time. This is believed to be an established method for making the desired perforations in graphene structures. The structures may be graphene nanoplatelets and graphene nanoribbons. Thus, apertures in the desired range can be formed by shorter oxidation times. Another more involved method as described in Kim et al. “Fabrication and Characterization of Large Area, Semiconducting Nanoperforated Graphene Materials,” Nano Letters 2010 Vol. 10, No. 4, Mar. 1, 2010 pp 1125-1131, utilizes a self assembling polymer that creates a mask suitable for patterning using reactive ion etching (RIE). A P(S-blockMMA) block copolymer forms an array of PMMA columns that form vias for the RIE upon redeveloping. The pattern of holes is very dense. The number and size of holes is controlled by the molecular weight of the PMMA block and the weight fraction of the PMMA in the P(S-MMA). Either method has the potential to produce perforated graphene sheets.
Chemical methods (such as oxidation or doping) of creating holes in graphene generally operate by nucleating defects in the graphene lattice and growing holes through bond breaking at these defects. Since the defect nucleation and hole growth occur simultaneously across the graphene, a wide range of hole sizes is created. It is difficult to control the oxidation process to simultaneously keep hole dimensions small (nanoscopic) and the hole distribution narrow.
The above-mentioned methodologies create nanometer sized holes in graphene, but the preponderance of holes created are not in the size range (below 10 nm diameter) required for applications such as desalination. Although the above methodologies are adequate at forming a desired size of hole, several of those methods do not consistently provide the same size holes. For example, an operation to form holes may generate some holes having a diameter of 1.2 nm and other holes having a diameter of 2.5 nm. Typically, methods such as oxidation that create holes via initial nucleation of defects in graphene followed by growth of holes yield a range of hole sizes because the nucleation and growth processes proceed at the same time. Holes that start growth (from nucleations) earlier in the process will end up larger than holes that start growing later in the process. When the process is finished, there will be a range of hole sizes. In applications based on filtration size sieving this wide of a variation in hole diameter can be unacceptable as the membrane will be unable to discriminate between molecules that are desired and molecules that are not desired. Therefore, there is a need in the art to perforate graphene and other atomically thin membranes with precisely sized holes or apertures at the nanometer level to achieve precise size-based filtration. There is also a need to generate such perforated graphene with methods that are scalable for mass production.